Over the years a number of methods and a variety of apparatus were developed to collect and analyze the mainstream smoke from a cigarette, but there was no simple and convenient apparatus to collect or analyze the sidestream smoke from a cigarette. Similarly, there was no method or apparatus to determine accurately the relative amount of sidestream smoke produced by a cigarette.
Pyrolysis of tobacco in a cigarette produces several resultant products, commonly lumped together as "smoke." One way to classify such products is to divide "sidestream smoke"--that which passes directly from the cigarette surface to the atmosphere--from "mainstream smoke"--that which is drawn lengthwise through the cigarette to the user by drawing a slight vacuum on the unlit end. Also, "smoke" consists of a mixture of constituents in both aerosol and gaseous form; the former is visible, and the latter is not, due to the size of the particles concerned. The present invention relates to the accumulation and measurement of the aerosol portion of the sidestream smoke, and the term "sidestream smoke" as used herein refers solely to that portion of cigarette pyrolysis products.
Source emission monitors are well known and used in industrial facilities to measure smokestack emissions from factories. However, the apparatus and methodology used therein are not suitable for the collection and measurement of sidestream cigarette smoke because of the differences in size and complexity, the nature of the emissions measured, the environment in which measurements are made, the necessity of distinguishing among different emissions, and the relatively constant rate at which emissions are produced. In addition, source emission monitors relate to making absolute measurements of the amount of emissions on a continuing and instantaneous basis. Such monitors may measure opacity or gaseous emissions, and each type must meet rigid performance standards to provide an accurate, absolute reading.
The present invention may be generally classified as measuring opacity, but because it provides relative measurements rather than absolute measurements, it is simple in design and operation, inexpensive, very reliable, and easy to operate and maintain. In general, opacity monitors measure the attenuation of a light beam traveling from a source to a light sensor at a remote point, with the attenuation of the light beam being primarily due to absorption or scattering of the light by the matter between the source and sensor. Light carries energy, and will therefore interact with any particles, such as smoke particles, that it may strike, as energy from the light is absorbed by the particles. Moreover, different colors of light have different wavelengths and will have different effects upon different particles in terms of transferring energy thereto. Low energy light, i.e. light having a long wavelength, may cause a molecule to rotate. Light of a higher energy may cause a molecule to vibrate, and light of a still higher energy may excite an electron in the molecule to jump into a new orbit. Accordingly, one must accurately control the wavelength of the light source or compensate for the vagaries of energy absorbtion to obtain absolute measurements of the properties of the measured aerosol.
The opacity of an aerosol is also a function of light scattering. For particles having a diameter of the same order of magnitude of a wavelength of light or larger, scattering may be by way of external reflection, refraction, internal reflection, or diffraction. Each of these may have a different effect, and there may also be different cumulative effects. Particles having a diameter substantially smaller than a wavelength of light will scatter light by a process referred to as dipole or Rayleigh scattering, which causes the electrons in the molecule to oscillate. The oscillating electron will radiate energy in all directions, which scatters light in all directions. Due to the differences in the way that different size molecules scatter different wavelengths of light, these phenomenon can become very important in studying the opacity of an aerosol.
These principles relating to the attenuation of light have been used to design transmissometers, which monitor the concentration of particles emitted from smokestacks or the like. A transmissometer measures opacity by projecting a beam of light across a smokestack and the amount of light transmitted therethrough is a measure of the opacity of the emitted aerosol. It provides a quantitative value related to the decrease of the transmitted light, and, in the past, has provided an absolute value proportional to the percent opacity of the aerosol.
Prior art transmissometers have usually been very complex. To insure their accuracy, specific design criteria have included limitations on spectral output or composition of the light beam, angle of observation of the photodetector assembly, calibration error, response time, frequency of sampling, and systems operational checks. These criteria were deemed necessary because there is no widely available independent method of checking the opacity of the aerosol.
Aerosol monitors which utilize light for measurement rely upon the Beer-Lambert law. It states that the transmittance of light through a medium that absorbs or scatters light is decreased exponentially by the product .alpha.cl, or EQU T=I/I.sub.o =e.sup.-.alpha.cl
where:
T=transmittance of light through the aerosol. PA1 I=intensity of the light energy entering the aerosol. PA1 I.sub.o =intensity of the light energy leaving the aerosol. PA1 .alpha.=attenuation coefficient. PA1 c=concentration of particles in the aerosol. PA1 l=distance the light beam travels through the aerosol.
Opacity is related to transmittance as 100%(l-T)=% opacity.
The design principles and techniques utilized with transmissometers were tested for measurement of the amount of sidestream smoke produced by a cigarette, but there were multiple problems and they were unsuccessful. For instance, a tungsten halogen light source had its light beam focused across a chimney to a photodetector. Sidestream smoke from a cigarette was permitted to rise through the chimney, and measurements were taken. The results were erratic and inconclusive due to the lack of concentration of sidestream smoke, wandering of the sidestream smoke out of the light beam, background "noise" being a significant portion of the output signal, and spiking of the output during puffs on the cigarette. The present invention solved these problems with the apparatus and methods disclosed herein, which collects substantially all of the sidestream smoke produced by a cigarette and measures the total amount thereof. The apparatus and method are simplified by making relative measurements rather than absolute ones, which essentially holds constant several of the factors that might otherwise adversely affect an absolute measurement.